Erosion, flooding and & pollution

We would welcome your contribution to this and other pages - If you have any info or questions, please email them to LeRoysBush@gmail.com

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Reporting erosion and pollution

If you see any signs of erosion or pollution, please see the contact details at Report pests, problems & pollution.

Please feel free to copy your email to leroysbush@gmail.com for our information.

Flooding, erosion and pollution problems in Le Roys Bush

Some erosion and flood damage problems we are aware of:

• • Upper Le Roys Bush - below Highbury - the stream banks have been collapsing

  • Below the Enterprise Street entrance to Le Roys

  • Around the upper wetland

  • Around Dudding Park and the boat yard

Flooding and sewage in upper Le Roys - impact on stream life

Scientific studies of the native fish in Le Roys Bush have shown significant populations of the native "trout" known as banded kokopu in the streams between the waterfall and Highbury over recent years. Up until late 2011, over 400 individual native fish had been identified living in the pools above the waterfall. The suddenly in February 2012, they nearly all disappeared. The cause has not been established - possibilities include sewage leaks, flash flooding, release swimming pool water and road or other pollution. Over 2013 and into 2014, the kokopu started to repopulate the streams and pools above the waterfall.

Sewage?

The team doing the studies had observed the smell of sewage in these areas but further investigation didn't find an immediate cause. A major sewage overflow from Highbury in November is reported on at Sewage pollution in Le Roys Bush 23 Nov 2014 - fortunately at least some of the kokopu survived this major release which ran for over 36 hours.

Flash flooding?

The team has also made informal observations of the flash flooding levels. The photo below shows a red toy stuck in a nikau palm well above ground level. The toy ‘lined up’ with other flood debris caught in nearby trees/palms.

This man in this photo is holding his hand at the level of the toy - indicating the estimated flood water level in the valley below "The Birkenhead" apartment block. These levels would flood parts of the new upper Le Roys walkway.

But more importantly - what is the impact of torrential stream flows like this on the ecology of the bush. And what can be done to mitigate flash flooding from the rooftops and pavements of Highbury? Does Council have the powers and/or budget to create rain gardens, swales or detention tanks to slow down and cleanse this stormwater?

Dudding Park, Rods Island and the boat yard

The sportsfield at Dudding Park and the area round Rods Island and the Bowling Club have been subject to floods for many decades. The flooding arises from a combination of two circumstances:

• • heavy stormwater volumes coming down the streams following heavy rains

  • king tides in Little Shoal Bay.

These floods often cover the sportsfield and bowling green; sometimes the road is under water.

However a number of people have observed that tidal inundation has got worse in recent years even in the absence of stormwater. High tides are causing salt water flooding of the sports fields and the raupo wetland - causing dieback of the grasses and raupo. Salt marsh plants are appearing at the western end of the Dudding Park sportsfield.

Arguably these high tides are caused by climate change and/or a combination of weather circumstances. If these king tides continue, then Council will have the option of allowing the reclaimed land in Dudding Park and around Rods Island to return to a salt tolerant plant regime or of undertaking major capital works. We hope that they will communicated with the community and stakeholders about this.

Actions Council has taken in the past

The North Shore City Council and the new Auckland Council have undertaken a number of actions to try to prevent further erosion and pollution over the years.

• • In ~2004, the Water Services department undertook a stream study - I can no longer find this on the North Shore City Council site. A walkabout has been repeated in 2012-2013 - but we have not seen a copy yet.

  • In 2009, the Water Services department planted a lot of carexes and other plants around the stream below the Westpac Bank building.

  • In 2010-11 the Council undertook a project in Seaview and Valley Road. See http://www.naturallysmarter.co.nz

Streams and flood plains

Le Roys Bush is subject to some heavy storm water flows resulting in erosion and the possible loss of native fish habitats.

The following screenshot shows the Auckland Council GIS map of storm water streams, flood zones and flood sensitive areas.

During 2012 and 2013, floodwater volumes appeared particularly heavy.

The disappearance of over 400 banded kokopu from the stream above the waterfall may have been due to pollution and/or heavy flooding.

Some native plantings undertaken by volunteers also appear to have been affected by heavy flooding.

Council officers advise us that increased water volumes may be due to more severe storms in this period and/or increased impervious paving.

Council is keen to reduce the volume of flooding and also the levels of pollution entering Le Roys and other bush reserves.

Enterprise Street

At the bottom of Enterprise Street, some major drainage works were undertaken in the early 2000s where there is public access beside Enterprise Motors.

On the slope here, Water Services planted some native trees - although they are struggling - not far below there has been major erosion which has disturbed the pools where kokopu breed.

Below is a photo taken in 2012/13 of what we assume is a drainage pipe may have been vandalised.

Swale at Onewa Road entrance to Le Roys Bush

The LRB Management Committee was invited in June 2014 to provide feedback on a proposed swale alongside the entrance path to Le Roys Bush. The swale is designed to carry stormwater currently running overland through private properties nearby. It will be on the west side of the track to channel overflow stormwater through a bed of suitable plants that will help to filter contaminants before the water enters the LRB stream.

This swale is only for excess flows - normal flows will flow through a stormwater pipe running further down Onewa Road.

We were interested to know where this pipe discharged to. We also wondered what impact the overall approach to storm water management would have on water levels in the bush and the streams. We have asked Council if it would be useful to have feedback from ecologists about the issue of managing water quality and flash flooding in Le Roys.